Mountain climbing ______ be really dangerous at times even though it is challenging and exciting in general.

— Who _____ be phoning us at this time of night? — It might be your sister. 

She ______ have attended that meeting, for she was doing paperwork in the office then.

We ________ the difficulty together, but why didn’t you tell me?

________ we go to the McDonald's? I'm sick of the food there.